First be sure, that your order is loaded ... take a look at:
protected function insertOrder(&$page, $obj, $putOrderId = true)
{
if ($obj instanceof Mage_Sales_Model_Order) {
$shipment = null;
$order = $obj;
} elseif ($obj instanceof Mage_Sales_Model_Order_Shipment) {
$shipment = $obj;
$order = $shipment->getOrder();
}
.....
}
So later you can use this snippet:
$invoiceIncrementId = '';
if ($order->hasInvoices()) {
// "$_eachInvoice" is each of the Invoice object of the order "$order"
foreach ($order->getInvoiceCollection() as $_eachInvoice) {
$invoiceIncrementId = $_eachInvoice->getIncrementId();
}
}
I referred, that forum reply: http://www.magentocommerce.com/boards/viewthread/198222/#t393368
Good luck.